Photo detecting in the near-infrared regime, e.g., wavelengths of between about 0.7 μm to 2 μm, has many applications, such as fiber-optical communication, security applications and thermal imaging. Silicon photodiodes are widely used as photodetectors for the visible light regime because of their low dark current and compatibility with silicon integrated circuit technologies. Attempts have been made to deposit Si1-xGex (SiGe) alloys, or germanium thin films, in silicon to fabricate an IR detector, however, the lattice mismatch in a SiGe-to-silicon boundary limits the maximum SiGe thickness which may be formed while still maintaining a defect free structure. Additionally, SiGe thin films having a germanium concentration of less than 50% have a relatively low IR absorption coefficient. Hence, it is difficult to fabricate an IR detector on SiGe film which has high sensitivity and low dark current.
Germanium has a high IR absorption coefficient, however, a 4% lattice mismatch to silicon results in a high dark current when a germanium photodetector is fabricated by direct deposition of germanium on silicon. A germanium photodetector having a simple PN diode structure exhibiting a very high dark current, e.g., about 30 mA/cm2, has been fabricated, Colace et al., Efficient high-speed near-infrared Ge photodetectors integrated on Si substrates, Applied Physics Letters, Vol. 76, No. 10, pp 1231-1233 (2000).
A germanium thin film bonded to silicon or silicon dioxide has been disclosed, wherein the IR sensor and the driving CMOS circuit are both fabricated on a germanium thin film. High temperature processes are required for germanium IC processes, i.e., ion implantation activation processes are usually performed following an annealing at about 800° C. Such high temperature processes degrade the quality of germanium thin films because the thermal expansion coefficient of silicon, germanium and silicon dioxide are different. A high temperature process performed on a germanium thin film which is bonded to a silicon wafer usually results in defects in the germanium layer(s), as described in U.S. Pat. No. 6,645,831, for Thermally stable crystalline defect-free germanium bonded to silicon and silicon dioxide, granted Nov. 11, 2003 to Shaheen et al. Additionally, germanium IC processes are not as well established in the semiconductor industry as are silicon IC processes.
A graded SiGe growth has been demonstrated to form a thick SiGe buffer layer prior to germanium growth, Lee et al., Optimized Strained Si/Strained Ge Dual-channel Heterostructures for High Mobility P− and N-MOSFETs, IEDM 2003, pp 429-432.
A Germanium p-MOSFET fabricated on bulk germanium and on epi-germanium has been disclosed by Ritenour et al., Epitaxial Strained Germanium p-MOSFETs with HfO2 Gate Dielectric and TaN Gate Electrode, IEDM 2003, pp 433-436.
In U.S. patent application Ser. No. 10/993,533, filed Nov. 19, 2004, for Fabrication of Thin Film Germanium Infrared Sensor by Bonding to Silicon Wafer, a germanium IR detector is described, which detector may be fabricated on a silicon CMOS circuit by a bonding process in which a germanium donor wafer is required. The size of commercial germanium wafers is smaller than that of silicon wafers, and the process is therefore unable to take advantages of larger silicon wafers.